The present invention relates to a resistance adjusting type heater which can be suitably used for purification of automobile exhaust gas, for example.
Porous ceramic honeycomb structures are presently used as, a catalyst, or a catalyst carrier, for conversion of nitrogen oxides (NO.sub.x), carbon monoxide (CO) and hydrocarbons (HC) present in the exhaust gas emitted from the internal combustion engines of automobiles, etc. Further, metallic honeycomb structures have drawn attention recently.
Meanwhile, with the stricter emission controls gas, it is desired to develop a heater or the like capable of reducing emissions at cold start of an engine is desired.
As a honeycomb structure as mentioned above, there Japanese Utility Model Laid-Open No. 67609/1988 utilizes a honeycomb structure as noted above. In this document, there is disclosed a catalytic converter comprising (a) a ceramic main monolith catalyst and (b) an electrically heatable metal monolith catalyst provided upstream of the ceramic main monolith catalyst (a) in close vicinity thereto. The metal monolith consists of a metal carrier and alumina coated thereon.
In the catalytic converter described in the document, the metal monolith catalyst thereto is a foil type metal honeycomb structure wherein electricity is passed from the inner periphery to the outer periphery for heating the structure. The metal monolith has no resistance adjusting means (that is, only the material, dimension and rib thickness are specified and no resistance control is made). Moreover, the metal monolith catalyst has substantially the same cross-sectional diameter as the main monolith catalyst; Therefore, the metal monolith suffers from insufficient heat-generation.
Hence, the present applicant proposed in U.S. Pat. No. 5,063,029 a heater comprising a honeycomb structure, at least two electrodes for enabling electrical heating of the honeycomb structure, provided on the honeycomb structure, and a resistance adjusting means provided between the electrodes. This heater is capable of controlling its heat-generating property and is useful for reducing automobile emissions at cold start.
However, when slits such as shown in FIG. 1 are used as resistance adjusting means in the above-described heater, the honeycomb structure 10 has a heat-non-generating portion 12 at the outer periphery 11. Consequently, exhaust which gas passes through portion 12 is not sufficiently or efficiently purified.